I have the stock nav system fitted in mine and the standard one i took out.

You should not have the nav button on the stock radio.
But the black button you mention should be an LED which flashes when the unit is off.
Do you get a compass arrow on the RHS of the display when the radio is on if so you do have the Nav system.
Have a look in the menu there should be plenty of nav options which you would not have on the stock radio.